1. Field of the Invention
The present invention relates generally to semiconductor device and fabrication method thereof. More particularly, the present invention relates to a dual work-function metal-gate CMOS device and fabrication method thereof.
2. Description of the Prior Art
The continued scaling of CMOS devices into sub-40 nm technology will rely on a fundamental change in transistor gate stack materials. Over the past few years, research in this area has focused on identifying candidate materials to replace polysilicon and SiO2 as the gate electrode and gate dielectric, respectively. Critical requirements for novel gate electrode materials include thermal stability with the gate dielectric and suitable values for the interfacial work function. The latter requirement of obtaining complementary gate work functions on a single wafer is being perceived as a major process integration challenge.
Metal-gate electrodes bring about several advantages compared to traditional polysilicon gates as CMOS technology continues to scale beyond the 40 nm node. These include reduction in poly-depletion effect, reduction in sheet resistance, and potentially better thermal stability on high-K gate dielectrics. The main challenge is that, unlike with polysilicon, one would have to use two metallic materials (bi-layer metal) with different work functions in order to achieve the right threshold voltages for both NMOS and PMOS.
The conventional dual metal gate methods are categorized into the gate-first process and the gate-last process. Among the two main approaches, the gate-last process is able to avoid processes of high thermal budget and to provide wider material choices for the high-K gate dielectric layer and the metal gate, and thus gradually replaces the gate-first process. In a conventional gate-last process, a dummy gate (or “replacement gate”) is formed on a substrate and followed by steps of forming a conventional MOS transistor and forming an inter-layer dielectric (ILD) layer. Subsequently, the dummy gate is removed to form a gate trench. Thereafter, the gate trench is filled with metal layers required for different conductivity types.
It is well known in the art that the degree of difficulty for fabricating a well-controlled double work function metal is immense as the process often involves complicated integration between NMOS device and PMOS device. The difficulty increases even more as the thickness and materials used in double work function metal gates requires a much more strict demand. Hence, how to successfully integrate the fabrication of a conventional double work function metal gate transistor so as to reduce its complexity to the standard CMOS process flow has become an important study in the field.